The present invention is directed generally to liquid crystals, and, more particularly, to improved low voltage operation of liquid crystal devices.
Liquid crystal display elements utilize the optical anisotropy and the dielectric anisotropy of liquid crystal materials. Various display modes and various driving methods for driving the display modes are well-known.
The properties of liquid crystal materials used for these liquid crystal display devices are various, but any liquid crystal materials have in common stability to moisture, air, heat, light, etc. Further, it is required for the devices that the liquid crystal phases have a temperature range as broad as possible, around room temperature, have a low viscosity, and, in the display device, have a quick response rate, a high contrast, and a comparatively low driving voltage. In addition, it is necessary that the liquid crystal materials have an adequate dielectric anisotropy (xcex94∈). However, a single liquid crystal compound satisfying these characteristics apparently has not yet been found. Thus, it is common to blend several different liquid crystal compounds and non-liquid crystal compounds to form a mixture that adequately meets the needs of the specific application.
A few high dielectric anisotropy dye compounds have been reported in literature. Examples include:
1. nitro-amino azobenzenes (I) and nitro-amino-tolanes (II) with structures shown below: 
published by S. T. Wu et al, Asia Display, pp. 567-70 (1995); and
2. bicyano-amino polyene dyes (III) with structures shown below: 
published by S. T. Wu et al, Japanese Journal of Applied Physics, vol. 37, pp. L1254-1256 (1998).
A general problem with these dyes is that their absorption is too large in the visible region, which causes these dyes to be colored. As a result, light transmission is greatly reduced. This is particularly undesirable for some displays and electro-optic modulators where high transmittance is required. Further, some of these dyes do not provide low voltage operation of the liquid crystal mixture.
Thus, there is a need for dyes that are colorless, have viscosities suitable for liquid crystal applications, approximately 20 mm2/s, and support low voltage operation, with a threshold voltage Vth of less than 2 Vrms.
In accordance with the present invention, a dopant is provided that is colorless, has a viscosity approximately the same as that of liquid crystal compositions, and supports low voltage operation. The dopants have one of three structures: (1) biphenyl; (2) diphenyl-diacetylene; and (3) double tolane, and in each case, have an amino group (secondary or tertiary) attached at one end of the molecule, with at least one polar group at the other end of the molecule. Schematically, the generic structure may be illustrated as: 
where R1 is an alkyl having from 1 to 12 carbon atoms, R2 is either hydrogen or, independently of R1, an alkyl having from 1 to 12 carbon atoms, M is a polar group, X and Y are independently hydrogen or a polar group not necessarily the same as M, and (A) is (1) a single bond (biphenyl); (2) xe2x80x94Cxe2x89xa1Cxe2x80x94Cxe2x89xa1Cxe2x80x94 (diphenyl-diacetylene); or (3) 
(double tolane), where Z is hydrogen, F, or alkyl.
Also in accordance with the present invention, one or more of the foregoing colorless dopants is added to a liquid crystal composition for supporting low voltage operation.
The teachings of the present invention provide new molecular structures of colorless dopant compounds for low voltage liquid crystal operation. These compounds exhibit an extraordinarily large dielectric anisotropy and relatively low viscosity. Therefore, adding a few percent of such dopant to a liquid crystal mixture reduces the operating voltage significantly while retaining fast response time. The low voltage operation enables use of a low cost electronic driver.